Quantification of water fluxes and soil water balance in agricultural fields under different tillage and irrigation systems using water stable isotopes

Sustainable agriculture should be based on management practices that improve resource usage efficiency and minimize harmful impacts on the environment while maintaining and stabilizing crop production. Both tillage and irrigation can have an influence on water resource usage and thus on hydrological processes within agro-ecosystems. However, it remains difficult to directly assess the effect of practices on water fluxes. Therefore, the objective of the study was to use oxygen and hydrogen isotopes (d18O, d2H) in the pore water of soil profiles as well as moisture contents for quantifying the soil water balance and fluxes. Covering all combinations, water content and isotope analysis in soil profiles were performed for 16 plots planted with winter wheat and managed with different tillage (conventional tillage (CT), reduced tillage (RT), minimal tillage (MT), and no-tillage (NT)) and irrigation systems (hose reel boom irrigation with nozzles (BI), sprinkler irrigation (SI), drip irrigation (DI) and no irrigation (NI)). The results indicated that the more intense the tillage, the lower the water content. Among the irrigation systems, DI had the highest average water content. Tracing the isotope minimum of winter precipitation in pore water of soil profiles showed deeper percolation of water in the CT plots, which indicates higher water flow velocity for CT compared to other tillage variants. Considering both water content and water flow velocities resulted in average water fluxes ranging from 23 to 46 mm for six months (end of November to May). For the same time period, between 80% and 91% of the water contributed to evapotranspiration. The resulting evapotranspiration within tillage and irrigation variants decreased in the order CT>RT>MT>NT and SI>BI>DI>NI. Thus, the method revealed that the lower water content in CT fields is a consequence of both deeper water infiltration and higher evapotranspiration. Moreover, irrigation water contributed mostly to evapotranspiration, and drip irrigation showed the lowest evapotranspiration among irrigation systems. This study demonstrated that water stable isotopes can be used as indicators and are a promising method to quantify average water fluxes in agricultural fields with great potential for evaluating management practices.